1. Field of the Invention
The invention relates to the separation and recovery of sulphur dioxide and unreacted monomer or mixture of monomers from the products obtained in the catalytic polymerization of acrylonitrile and copolymerization of acrylonitrile with further ethylenically unsaturated monomers copolymerizable therewith.
2. Description of the Prior Art
It is known that polyacrylonitrile and copolymers of acrylonitrile containing the latter in a proportion of at least 85 percent by weight are widely used in the art, more particularly for the production of fibers. These polymers are prepared in the art by polymerization of acrylonitrile or by copolymerization of the latter with one or more ethylenically unsaturated compounds using suitable catalysts and normally operating in an aqueous medium. The best known catalysts for the purpose are the redox systems in which a percompound is used in combination with a reducing substance usually consisting of a sulphite or sulphur dioxide.
As is known, when polymerizing or copolymerizing acrylonitrile, complete or practically complete conversion of the monomers is not obtained. Moreover, the monomer conversion value is often maintained lower than those practically attainable, thereby to confer special properties to the acrylonitrile polymers. Therefore, the problem of separating and recovering the unreacted monomers from the reaction medium arises. Recovery of the monomers is as a matter of fact a factor considerably affecting the economy of a commercial process for preparing acrylonitrile polymers. Moreover, in such polymerization processes it is desirable to recover sulphur dioxide from the reaction medium, both in order to avoid its presence in wastes, and to re-utilize it in the polymerization process.
According to one widely exploited technique, the polymer slurry is filtered on completion of polymerization, the monomers being separated from the filtrate by distillation.
This procedure suffers from drawbacks due above all to monomer losses at the filtering step and to a possible polymerization at the recovery section of the apparatus, where a higher monomer concentration is present. Moreover, the process is expensive.
These drawbacks are avoided or at least substantially reduced by the present process of the invention which relates to the separation and recovery of the monomers and sulphur oxide from the polymer or copolymer slurry obtained by polymerization or copolymerization of acrylonitrile.